Method of manufacturing polyamides by alkaline polymerization of cyclic lactams

ABSTRACT

An improved process for manufacturing polyamides by alkaline polymerization of cyclic lactams which consists of reacting alkali metal hydroxides or alcoholates with the lactams, with water or the corresponding alcohol being removed and putting the reaction mixture into contact with molecular sieves.

nlted States Patent [151 3,674,751

Kralicek et al. July 4, 1972 [54] METHOD OF MANUFACTURING [56]References Cited POLYAMIDES BY ALKALINE UNITED STATES N POLYMERIZATION0F CYCLIC TS LACTAMS 2,882,244 4/1959 Milton ..252/455 0 0 7 72Inventors: Jaroslav Kralicek, Praha; Vladimir Ku- 9 3 5/1963 Paper) eta]banek, Kralupy; Jitka Solcova; Jaroslava Kondelikova, both of Praha, allof j i 3 m l z g Czechoslovakia ssistant xammeraynes Attorney-Arthur O.Klein [73] Assignee: Zavody Antoniza Zapotockeho, narodni podnik,.laromer, Czechoslovakia [57 ABSTRACT [22] Fil d D 22, An improvedprocess for manufacturing polyamides by alkaline ol merization of c cliclactams which consists f 21 A LN .1 887,399 P Y Y 1 pp 0 reacting alkalimetal hydroxides or alcoholates with the lactams, with water or thecorresponding alcohol being removed b and putting the reaction mixtureinto contact with molecular n g sieves [58] Field ofSearch ..260/78L 7Claims, No Drawings where X is a polar substituent, or substancescapableof forming said activated carbonyl group as activator, saidcatalyst being prepared in situ by reacting an alkali metal hydroxide oralcoholate of lower molecular weight than the lactam used, the inventionconsisting in removing the low molecular weight by-product of thereaction, i.e.,either water or a lower alcohol by means of molecularsieves at temperatures above the melting point of the lactam employed orits solution and preferably underl50 C., prior to adding an activatorand starting the rapid polymerization. As molecular sieves syntheticzeolites having effective pore diameter of 3A., 4A., 5A., 6A. are mostsuitable for the purposes of the invention.

In the field of anionic (alkaline) polymerization of lactams severalimportant improvements were reported during the last decade. Thedeciphering of the reaction mechanism thereof resulted in development ofsome new progressive technological processes,-making use of the highpolymerization rate. The first success involved the polymerization of6-caprolactam. It has been, however, suggested also to polymerize in theabove mentioned way other lactams with'less than seven atoms in thering, e.g., pyrrolidone', and also higher lactams as, e.g., 7-enantholactam, 8-caprylolactam, IO-caprinolactam or l2-laurolactam ortheir substituted homologues and derivatives such as C-alkyl lactams; Acopolymerization wasalso considered.

Low content of the monomer, suitable physical properties, easy controland good reproducibility of the process were favorable forthedevelopment of anionic rapid polymerization of lactams, particularlyusing the so-called polymerization casting below themelting point of thepolyamide thus formed. Said method makes possible the manufacture ofarticles of any size and shape, requiring almost no finishing.Continuous processes of this kind were used either for'manufacture ofgranular polyamide as an intermediate-forfinal working, e.g., pressingor extrusion, or for direct-manufacture of endless profiles of varioussorts, if the rapid polymerization process was combined with extrusion.

Usual catalysts employed today comprise systems with at least twocomponents. The main component of all known and used systems forms astrong'base, usually calledcatalyst," while the other component(activator") is a substance with an activated carbonyl group in a lactammolecule, of the type wherein X is a polar substituent, or a substancecapable of forming said structure in situ. Examples of such activatorsare N-acyl lactams, N-alkyl diacyl amines, isocyanates, and chlorides,anhydrides or esters of carboxylic acids.

The search for improvements has concentrated in recent years on theactivator component.

lt has been found, however, that for securing a reproducible process itis also important to have the catalyst, i.e., the strongly basiccompound, in a pure state, its method of preparation being thus equallyimportant. As end product in the preparation of the basic catalyst thecorresponding salt of the lactam involved is to be considered. The saltis at least partly dissociated in the molten lactam to yield a lactamanion and metal cation. The type and concentration of the strongly basiccatalyst determines,toge ther with the activator, the rate ofpolymerization. lts influence on the molecular weight of the polymerformed is, however, negligible at comparatively low temperatures,employing the usual concentration of the activator. Some methods ofpreparing anionic catalyst are less I suitable because the reactions ofbasic compounds with lactams .are accompanied by undesiredside-reactions forming bysproducts which decrease the activity of thecatalyst. For instance, metallic sodium yields not only the sodium saltof -caprolactam but also 'hexamethylenimine, 6-aminohexanol and water,said by-products having an unfavorable influence on the polymerizationrate and the molecular weight of the product.

One of the most often recommended and economically and technologicallyadvantageous methods of preparing basic catalysts is the reaction of thelactams with alkali metal hydroxides. The reaction 00 CO e 112).. M0 H2Omixture, e.g., by distilling it from the top fractions of the lactam orby stripping it out by means of a stream of inert gas. The presence ofwater in the reaction mixture is to be avoided because water is a veryactive'inhibitor of polymerization, hydrolyzing imide groups which aregrowth centers, and also amide groups with simultaneous decrease ofalkalinity. The hydrolysis of the amidic group of the lactam moleculeforming the most important side-reaction in the preparation of thecatalyst may be suppressed by choosing suitable reac tion conditions,particularly by decreasing temperature and thus-also the pressure andthe rate of distillation. The best results can be reached only at apressure lower than 3 to 4 Torr at the corresponding boiling point ofthe lactam. Even at such conditions the yield of the alkali metal saltof 6-caprolactam is not fully quantitative because of the hydrolyticsidereaction. Moreover, the regeneration of the lactam, containedtogether with water in the top fractionin an amount up to 10 percentof'the total amount ofthe lactam, is in any case to be considered. Thepreparation of alkali metal salts by this method is thus very exactingasto the maintenance of .proper reaction conditions, the use of highlyeffective vacuum pumps being necessary. The removal of water from thereaction mixture and simultaneously the decrease of temperature can bemade easierbyadding solvents, thus forming azeotropic mixtures withwater. This method has certain disadvantages in difficult removal oflast traces of the solvent and the isolation of a fine crystalline saltis not entirely easy in most cases. The precipitated salt is rathergel-like in characteristic.

The preparation of alkali-metal lactam salts by the reverse reaction oflactams with alcoholates:

(0111):: ROMo 00 e om)n M0 non wherein R is a lower alkyl, requiresdistilling off the alcohol formed at reduced pressure together with thetop fraction of vents or melts, i.e., from substantially static systemsis known but their use for dynamic systems where said low molecularsubstances are liberated as a result of chemical reactions has not yetbeen hithertofore disclosed.

It has been found that molecular sieves, e.g., of the type of syntheticzeolites with a suitable size of crystalline interstices may be used forremoving water or the lower aliphatic alcohols such as methanol, ethanolor isopropanol originating in the above mentioned reverse reactions. Assuitable absorbents all molecular sieves with a minimum size ofinterstices, corresponding to the critical size of the water or alcoholmolecules proved useful with optimum results being achievable if thesize of said interstices was not larger than the size of the lactammolecule. Suitable products are, for instance, Nalsit 4A, Calsit 5A,Potasit 3A, Linde 3A, Linde 4A, Linde 5A (Registered Trade Marks), orother similar types of various products.

The removal of water or alcohols liberated during the reaction by aphysico-chemical method has many advantages in comparison with the abovediscussed known method. The most important is the use of a very simpledevice and a low reaction temperature, limited by the melting point ofthe monomer on one hand and by decreasing absorption capacity ofmolecular sieves with increasing temperature on the other. Even when,for example, the molecular sieve 4A adsorbs at 150 C. more than 5percent by weight of water it is apparent that a substantially lowertemperature determined by the melting point of the lactam would be moreadvantageous considering the rather high rate of hydrolysis of the amidegroup by the hydroxide at high temperature and considering also thesubstantially higher adsorption capacity at lower temperatures. Fromthis standpoint the new method will be advantageous particularly in thecase of lactams with low melting point such as 4-pyrolidone,S-piperidone, 6-caprolactam, 7- enantholactam and 8-caprylolactam andtheir C-alkyl substituted derivatives.

The new method may be used, however, also for preparation of salts ofhigher melting lactams, e.g., of l2-laurolactam; but then it ispreferably carried out in the presence of suitable inert solvents suchas aromatic hydrocarbons, ethers and similar materials, thus making itpossible to carry out the reaction in a solution at room temperature oronly slightly increased temperature.

In view of the fact that the process is always controlled byestablishing the reaction equilibrium and simultaneously by thediffusion rate of water or alcohol into the particles of the molecularsieve, it is clear that the duration of the reaction is dependent ontime.

The optimum time depends on the temperature, on the kind of monomer andhydroxide and their concentrations and also on the kind of sieve and theexperimental operation thereof. Therefore, the time is to be establishedindividually, and specifically for each system. In most cases therequired time of preparation in continuous operation is less than anhour.

The final product of the process is a solution of the lactam salt inmolten lactam, and is utilizable immediately for polymerization. Fromthe point of view of experimental or technological operation, it ispossible to use either a discontinuous or a continuous process, thelatter being particularly advantageous for pilot plant or industrialscale units.

For discontinuous or batch type operation, it is possible to meter intothe melted lactam either the molecular sieve and a hydroxide(alcoholate) simultaneously or the two components separately, themolecular sieve been preferably added first. The amount by weight ofthemolecular sieve is selected by its capacity vs. its dependence on thetemperature, by the water content of the lactam as well as by the amountof water or alcohol liberated by the reverse reaction. The total amountof water or alcohol must always be lower than the total capacity of themolecular sieve used. The process may be accelerated and the quality ofthe alkali metal lactam salt enhanced by stirring the reaction mixtureor by circulating it in a closed space through a layer of the molecularsieve. After the reaction is finished, the resulting solution of thelactam salts in the melted lactam is separated from the molecular sieveby any suitable means, e.g., by filtration. While the discontinuousprocess is most suitable for small units or for preparation of somespecial types of polymers, the continuous process is more appropriatefor an industrial process where the reaction mixture, containing asolution of an alkali metal hydroxide in molten lactam flows through alayer of the molecular sieve, e.g., in a column heated up to the meltingpoint of the lactam, the resulting clear solution of the lactam salt inthe lactam being led directly into the polymerization apparatus.

After the adsorption capacity of the sieve has been ex-- hausted, it issufficient to wash the sieve out with suitable solvents such as withwater, alcohols, ketones, hydrocarbons, chlorinated hydrocarbons orsimilar substances and to heat the sieve up to the prescribedtemperature, if desired at reduced pressure or in a stream of an inertgas, whereby the sieve is regenerated. The regeneration of the sieve maybe carried out directly in the column in which the lactam salt solutionis being prepared. The exhausted adsorption capacity of the sieve may beindicated in a usual, known manner, for instance, by a change of colorof the sieve.

As is apparent from the preceding disclosure it is possible to preparesolution of salts of practically all cyclic lactams capable of anionicpolymerization. The resulting reaction mixture may be immediately usedfor polymerization. In addition, with simple lactams of the formulawherein n is an integer from 2 to 14, it is possible to use alsosubstituted lactams such as C-alkyl lactams or also mixtures of variouslactams whereby copolymers may be prepared. The suggested method may beapplied also to the preparation of a monomer mixture containing inaddition to monolactams also lactams with a higher functionality, e.g.,bis-lactams, derived from corresponding bis-cyclohexanones (such asbis-caprolactam, methylene-bis-caprolactam, methylene-bis-caprolactam,ethylidene-bis-caprolactam, tetramethylene-bis-caprolactam,izopropylidene-bis-caprolactam and similar).

For the purpose of the invention various hydroxides or alcoholates ofalkali metals or alkaline earth metals such as sodium, potassium,cesium, rubidium, lithium, and barium may be used. Said compounds may bedissolved prior to the reaction in the lactam as such, or it is alsopossible to use their solutions in water, in alcohols or,advantageously, in inert solvents. With respect to full and economicalutilization of the molecular sieve the first 'altemative is better; inorder to decrease the time necessary for dissolution and for reducingthe possible hydrolysis it is advantageous to use finely groundhydroxides.

The herein disclosed use of molecular sieves for preparing reactionmixtures suitable for polymerization has also the advantage thatmolecular sieves remove from the lactam some accompanying low molecularimpurities, as so far as the size of the molecules corresponds to thesize of the interstices in the sieve.

The quality of the salts prepared in this manner is comparable with thequality of the salts prepared from alcoholates of alkali metals usingmethods suitable for exact study of kinetics and mechanism of anionicpolymerization. This has been proved by identical polymerization tests.The very good quality of the polymerization mixture prepared by means ofmolecular sieves results particularly from the low reaction temperatureused which suppresses undesired side reactions. Further advantages ofthe new method is to be seen in the possibility for using lactams ascurrently marketed without removing the humidity adsorbed during themanipulation by distilling the lactam immediately before use. Thenecessary equipment for utilizing the invention is comparatively simple,whereby the economy of industrial production of polyamides from cycliclactams can be enhanced. The new method also makes possible themanufacture polyamides in essentially non-chemical plants.

EXAMPLE 1 Fifty grams of the molecular sieve (Registered Trade MarkNalsit 4A) were added to the melt of 3 moles of freshly EXAMPLE 2 Theprocess according to Example 1 was repeated except that undistilled,technically pure 6-caprolactam was used together with a regeneratedsieve of the same kind. The obtained polyamide (half-time ofpolymerization being 12 minutes) contained 3.4 percent of low molecularcompounds and had a polymerization degree of 465.

EXAMPLE 3 One-hundred and fifty grams of a molecular sieve (RegisteredTrade Mark Calsit 5A) were added to 4 moles of melted 8-caprylolactamunder an argon atmosphere at 90 C., whereafter 0.01 mole of potassiumhydroxide, was added with stirring. After 15 minutes 423 g of thepotassium salt solution in the lactam were pumped into an adiabaticreactor and heated therein to 1 10 C. After addition of 0.0075 mole ofN- benzoyl-caprolactam the reaction was completed almostinstantaneously. The cooled polymer contained only 1.4 percent of lowmolecular weight compounds.

EXAMPLE 4 The process according to Example 3 was repeated except thatinstead of 8-caprylolactam, 7-enantholactam was used, the molecularsieve Calsit 5A being replaced by molecular sieve Linde 4A (RegisteredTrade Marks). The resulting polyamide contained 1.3 percent of compoundsextractable with water.

EXAMPLE 5 Fifty grams of a molecular sieve Nalsit 4A (Registered TradeMark) and 0.0075 mole of sodium methanolate were added to the 1 mole ofmelted 6-caprolactam and 0.5 mole of 12-laurolactam maintained underinert atmosphere at 100 C. After 15 minutes of stirring the mixture wasdivided into small polymerization apparatuses (21 g for each), and thepolymerization was carried out at 180 C. after there being added 0.00075mole of N,N-tetraacetyl hexamethylenediamine into, each charge. Theresulting polymer contained 3 percent of low molecular compounds.

EXAMPLE 6 0.45 g of bis-caprolactam, prepared from 6,C-bis-cyclohexanone, was dissolved in 33.9 g of melted caprolactam in aglass cylinder of 50 ml volume maintained at a temperature of 85 C.while stirring. Ten grams of the sieve Nalsit 4A (Registered Trade Mark)and 0.00076 mole of potassium hydroxide were then added through a sideinlet tube. After 10 minutes stirring at 85 C., 22.8 g of the mixturewere discharged into a small glass polymerization apparatus and thepolymerization was carried out by adding 0.0005 mole ofN-acetylcaprolactam and heating to 250 C. under inert atmosphere. After6 minutes the polymer was cooled down; it contained 10 percent of lowmolecular substances and was insoluble in mcresol.

EXAMPLE 7 Fifty grams of Nalsit 4A molecular sieve (Registered TradeMark) were added to 3 moles of 6-caprolactam containing 0.0075 mole of'y,'y'-bis-caprolactam at 95 C. and after a while 0.0075 mole ofpotassium methanolate was stirred in to the mixture. After 30 minutesthe reaction mixture was separated from the sieve and pumped into anadiabatic reactor, where 0.0075 mole of N-acetylcaprolactam were addedafter the mixture was heated up to 150 C. The polymerization wascompleted in 10 minutes. The polymer contained 5.4 percent ofextractables and was insoluble in m-cresol.

EXAMPLE 8 One-hundredth mole of potassium methanolate was dissolved atC. in a solution of 1 mole caprolactam in ml of benzene whereafter 30 gof Nalsit 4A molecular sieve were added while stirring. After 30 minutesthe solution was introduced in a three-necked flask, provided with areflux cooler, thermometer and stirring device. One-hundredth mole ofN-phenyl carbamoyl caprolactam was gradually added at 80 C. After 100minutes, 8 percent by weight of powder-like polymer separated.

EXAMPLE 9 A cylindrical glass column, 1,000 mm long and with a diameterof 60 mm, heated to 75 C. by means of a heating jacket was filled upwith molecular sieve Nalsit 4A. A solution of 0.3 mole percent of sodiumhydroxide in fi-caprolactam, maintained at the same temperature, waspoured through the column. From the bottom of the column the solution ofsodium salt of caprolactam in caprolactam was continuously drawn off.The catalyst solution thus obtained was used for a series of experimentsfor adiabatic polymerization in molds, the samples being heated to andC. respectively and 0.3 mole percent of N-acetylcaprolactam addedthereto. The properties of the polymers obtained were the same as whenthe polymers were prepared with pure crystalline sodium salt of-caprolactam at equal concentration of the components. In another seriesof experiments the catalyst solution with added, activator (0.3 molepercent of N- acetylcaprolactam) was metered into a tubular reactor forcontinuous manufacture and the molten polymer was extruded in the formof a string (monofil) with 1 mm diameter. The content of extractablesamounted to 10.2 percent. In a third series of experiments, the catalystsolution from the column was poured into metallic containers (made ofaluminum or stainless steel) and thereafter, the containers were sealedunder inert atmosphere. The polymerization activity of the mixturedecreased after 2 months only by 10 percent.

What is claimed is:

1. In a method for preparing polymerizable mixtures for anionicpolymerization of lactams selected from the group consisting of Iwherein n is an integer from 2 to 14, C-alkyl lactams, biscaprolactam,methylene-bis-caprolactam, methylene-biscaprolactam,ethylidene-bis-caprolactam, tetramethylene-biscaprolactam, andizopropylidene-bis-caprolactam, by reacting a compound selected from thegroup consisting of alkali metal hydroxides and alkali metal alcoholateswith said lactams and removing water or alcohol respectively, theimprovement step which comprises contacting the reaction mixture withmolecular sieves of synthetic zeolite type.

2. A method for preparing polymerizable mixtures for anionicpolymerization of lactams according to claim 1, in which the reactionmixture is contacted with a molecular sieve of synthetic zeolite typehaving an effective pore diameter from 3 to 6A., and separating theresulting lactam alkali metal salt solution in lactam from the molecularsieve.

3. A method for preparing polymerizable mixtures for anionicpolymerization of lactams according to claim 1 in which the lactam isreacted in the molten state.

anionic polymerization of lactams according to claim 1 in which thelactam is reacted with an alkali metal alcoholate.

7. A method for preparing polymerizable mixtures for anionicpolymerization of lactams according to claim 1 in which the molecularsieve has an effective pore diameter from 3 to 6 A.

2. A method for preparing polymerizable mixtures for anIonicpolymerization of lactams according to claim 1, in which the reactionmixture is contacted with a molecular sieve of synthetic zeolite typehaving an effective pore diameter from 3 to 6A., and separating theresulting lactam alkali metal salt solution in lactam from the molecularsieve.
 3. A method for preparing polymerizable mixtures for anionicpolymerization of lactams according to claim 1 in which the lactam isreacted in the molten state.
 4. A method for preparing polymerizablemixtures for anionic polymerization of lactams according to claim 1 inwhich the lactam is reacted in a solution.
 5. A method for preparingpolymerizable mixtures for anionic polymerization of lactams accordingto claim 1 in which the lactam is reacted with an alkali metalhydroxide.
 6. A method for preparing polymerizable mixtures for anionicpolymerization of lactams according to claim 1 in which the lactam isreacted with an alkali metal alcoholate.
 7. A method for preparingpolymerizable mixtures for anionic polymerization of lactams accordingto claim 1 in which the molecular sieve has an effective pore diameterfrom 3 to 6 A.